A major program has been launched on the nature of the reactions and enzymes of microsomes of liver, kidney, and intestinal mucosa of rachitic chicks. A new reaction that converts 25-hydroxyvitamin D3 to detectable but unidentified products has been observed. The new reaction was found to depend upon cytochrome P-450. Gross differences in the ratio of cyt. P-450 to cyt. b5 were found in the microsomes of the tissues examined. This ratio is being examined as a factor in regulating vitamin D metabolism in various tissues. Another study concerns the role of plasma proteins in translocating vitamin D metabolites not only across cell membranes but also across intracellular membranes of organelles. Preliminary results suggest that the receptor for 25-OH-vitamin D3 in kidney cytosol is identical to the plasma transport protein on the basis of reaction with antibody. Purification of the kidney receptor has been carried out and characterization by SDS electrophoresis has indicated that it is a polymer of identical subunits. Further characterization of the kidney and plasma proteins is planned, including amino acid analysis, to clarify the mechanism of transport of 25-OH-vitamin D3 into the cells that convert this compound into the physiologically active hormone.